In the average adult man, the small intestine is about 6 to 7 meters long, with a diameter of 2.5 to 3.0 centimeters. Its wall contains layers of smooth muscle and nervous tissue, which allow contraction movements and their control; its inner layer consists of absorptive mucosa, rich in blood and lymph vessels that drain into the portal vein. The mucosa is lined with absorptive epithelium.

Salem Press Encyclopedia of Science, 2023. 2p.